80386 Memory Pipeline
a day ago
- #FPGA
- #80386
- #Memory Pipeline
- The 80386's memory pipeline optimizes virtual memory access through overlapping operations, pre-calculation, and parallelism, reducing common-case address translation to about 1.5 clock cycles.
- Segment descriptor caching avoids repeated table lookups, with hidden state stored in segment registers for real and protected modes, enabling 'unreal mode' tricks by manipulating limits.
- Early Start optimizes address generation by beginning work in the last cycle of the previous instruction, overlapping with writeback, improving performance by about 9% but causing corner-case bugs like POPAD.
- Paging efficiency relies on a TLB for fast hits and a hardware page walker for misses, integrated into the memory pipeline without heavy microcode involvement.
- The bus interface supports non-multiplexed address/data buses and pipelining, while external caches (e.g., Intel 82385) significantly boost performance by reducing wait states.
- Mapping the 80386 pipeline to an FPGA involves adapting latch-based design and two-phase clocks to synchronous FPGA logic, using L1 caches for low latency instead of external caching.